Wood and wood based products have properties that may be improved to improve the performance characteristics of wood, and to extend their application to new fields, including, for example dimensional instability due to the hygroscopic nature of wood, discoloration, biological degradation and degradation on exposure to UV light.
Many of the conventional leading technologies for treating wood or wood based materials comprise impregnation with heavy metal preservatives. However, the utilization of heavy metals in preservatives for timber treatment is now not favoured due to toxicity and environmental issues.
Acetylation of solid wood has been extensively investigated in the past, but due to the lack of an appropriate processing system to impregnate, react and recover the by-product of impregnation in a time period to be feasible, its commercial application has been limited. However acetylation is environmentally friendly and can improve all or some of the aforementioned shortcomings of wood
Acetylation of the wood prevents water molecules from penetrating the cell wall by bulking the cell wall and substituting the hygroscopic hydroxyl groups of lignin and hemicellulose with hydrophobic (less polar) acetyl groups. Acetylation treatment can improve dimensional stabilization and biological resistance, and may also improve resistance to weathering effects, discoloration, and UV degradation.
Acetylation of solid wood is traditionally practiced in several steps. Typically, impregnation is first performed in a pressure cylinder and the acetylation reaction is then performed in a separate dedicated plant to heat and reflux the acetic anhydride for a lengthy period until acetylation is complete. After treatment and heat reaction, the by-product, which is a mixture of unreacted acetic anhydride and acetic acid, is recovered.
European patent 0 213 252 discloses a process in which wood was “impregnated by acetic anhydride using a vacuum or vacuum pressure technique, the material was then drained of excess acetic anhydride and placed in a chamber heated to 120° C. The material was heated at this temperature for 2 to 8 hours”.
European patent 0 680 810 A1 and B1 discloses a process in which wood was impregnated by acetic anhydride at a temperature in the range of 80-150° C. preferably 90-130° C. and then maintained at a temperature between 70-150° C. for a period of time up to 24 hours (suitably about 3 hours) to cause the acetylation reaction within the wood.
The effect of different catalysts on the reaction rate has also been investigated, but heating to temperatures of up to 139° C., which is the boiling point of acetic anhydride, has proved to be the best option.
Gaseous acetylation of solid wood achieves insufficient loading or weight gain for improvement in stability and decay resistance. To achieve a 15% weight gain in acetylation theoretically 30% or more acetic anhydride is required. To impregnate gaseously this amount of acetic anhydride in solid wood requires an extremely long impregnation time. Gaseous acetylation of wood at high temperature has only been successful on very small wood particles such as fibers, flakes, chips or sawdust. U.S. Pat. No. 6,376,582 discloses use of vapour of the acetylating agent at a reaction temperature of around 140 to about 210° C. at atmospheric pressure. Thus the gaseous acetylation of solid wood is restricted to very small wood dimensions as the rate of vapour-phase acetylation is determined by the rate of diffusion of vapours into wood and the acetylation time rapidly increases with the increase in the wood thickness. Application of gaseous acetylation has therefore remained restricted to thin veneers or solid wood only a few inches long in the fiber direction, in relation to which acetylation can be achieved within a practical treating time.
Traditional acetylation processes that heat pre-impregnated, saturated wood to react the acetic anhydride have poor heating efficiency due to the following problems:                1: The mass of acetic anhydride in the wood.        2: The poor heat conductivity of wood.        3: The fact that the wood heating is by conduction rather than convection.        4: Heat distribution is not uniform throughout the solid wood and thus acetylation is uneven.        5: The total treatment, reaction and recovery period required is extremely long.        
It is an object of the present invention to address at least some of these aforementioned shortcomings or to provide the public with a useful choice.